This invention relates to microelectronic devices and fabrication methods therefor, and more particularly to light emitting devices, such as light emitting diodes (LEDs) and manufacturing methods therefor.
Light emitting diodes are widely used in consumer and commercial applications. As is well known to those having skill in the art, a light emitting diode generally includes a diode region on a microelectronic substrate. The microelectronic substrate may comprise, for example, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new lighting applications, and may place LEDs in a position to compete with the well entrenched incandescent and fluorescent lamps.
Gallium Nitride (GaN)-based LEDs typically include an insulating or semiconducting substrate, such as silicon carbide (SiC) or sapphire, on which a plurality of GaN-based epitaxial layers are deposited. The epitaxial layers include an active or diode region having a p-n junction which emits light when energized.
LEDs may be mounted substrate side down onto a submount, also called a package or lead frame (hereinafter referred to as a “submount”). In contrast, flip-chip mounting of light emitting diodes involves mounting the LED onto the submount with the substrate side facing up (i.e. away from the submount). Light may be extracted and emitted through the substrate. Flip chip mounting may be an especially desirable technique for mounting SiC-based LEDs. In particular, because SiC has a higher index of refraction than GaN, light generated in the active or diode region generally does not totally internally reflect (i.e. reflect back into the GaN-based layers) at the GaN/SiC interface. Flip chip mounting of SiC-based LEDs also can improve the effect of certain substrate-shaping techniques known in the art. Flip chip packaging of SiC LEDs may have other benefits, such as improved heat dissipation, which may be desirable depending on the particular application for the LED.
One potential problem with flip-chip mounting is that when an LED is mounted on a submount using conventional techniques, a conductive die attach material such as silver epoxy is deposited on the LED and/or on the package, and the LED and the submount are pressed together. This can cause the viscous conductive die attach material to squeeze out and make contact with the N-type substrate and/or layers in the device, thereby forming a connection that can short-circuit the p-n junction in the active region.
Metal-metal bonds formed by soldering, thermosonic scrubbing and/or thermocompression bonding are alternative attach techniques. However, tin (Sn) is a component of most types of solder, and migration of Sn from the bonded surface into the device can cause unwanted degradation of the device. Such migration can interfere with metal-semiconductor interfaces such as ohmic contacts and/or the function of metal-metal interfaces such as reflective interfaces that serve as mirrors.
Semiconductor light emitting devices, such as LEDs, may be first attached to a submount and then the submount may be mounted to a substrate. So as to distinguish the substrate to which the submount is mounted from the semiconductor substrate on which the light emitting device is fabricated, the substrate to which the submount is mated will be referred to herein as a “mounting substrate.” The mounting substrate may, in turn, be mounted to a circuit board or other electrical circuit depending upon the particular application for the LED. Each of these connections typically includes both electrical and physical connection aspects.
A common form of connection used for mounting an LED to a submount and a submount to a mounting substrate is soldering using, for example, a die solder process. Soldering may be used to provide a metal-to-metal connection providing mechanical mounting, a thermal pathway and an electrical connection. Solder materials may generally be categorized as low temperature, medium temperature and high temperature solders. The temperature categorization may be based on the eutectic temperature of a particular composition. For example, a solder paste with particles of a silver-tin (AgSn) alloy with eighty percent (80%) silver generally has an associated solder temperature of about 220 degrees centigrade (° C.), making it a medium temperature solder material. A high temperature solder paste can include, for example, gold-tin (AuSn) particles having a solder temperature of about 280° C. A low temperature solder paste can include, for example, lead-tin (PbSn) particles having a solder temperature of about 180° C.
It is generally preferable not to reflow a previously formed solder connection. Thus, in applications involving multiple solder reflow process steps, different temperature solder materials may be selected so that subsequent reflow process steps do not detrimentally impact previously formed solder bonds. For example, it is known to use a high temperature solder paste to connect an LED to a submount in combination with a medium temperature solder paste for connecting the submount to a mounting substrate. A low temperature solder paste may then be used for connecting the mounting substrate to a circuit board. Thus, each subsequent process step may be carried out at a temperature below the reflow temperature for the previously formed solder connection. However, it is also known to use a high temperature solder paste to connect an LED to a submount and then a high temperature solder paste to connect the submount to a substrate and then a medium temperature solder paste to connect the assembly to a circuit board and a low temperature solder paste to connect the circuit board to a mother board.
It is also known to use a medium temperature solder paste for the LED to submount connection in combination with a low temperature solder paste for the submount to mounting substrate connection. The mounting substrate may then be connected to the circuit board using a low temperature solder paste but in a manner that reduces the heating of the submount to mounting substrate connection to avoid reflow of that connection in the later process. For example, the mounting substrate may be mounted to the circuit board using standoffs providing thermal isolation from the heat used to solder the standoffs to the circuit board. Similarly, it has been proposed to use leads offset to the side from the submount to provide thermal isolation of the LED to submount solder bond from the heat generated when soldering the submount leads to the circuit board. However, these various approaches may not be compatible with higher density circuit board mounting techniques, particularly in surface mount applications that are generally used where higher density mounting of components on the circuit board is desired.